1. Field of the Invention
Embodiments of the present invention generally relate to semiconductor processing equipment. More particularly, embodiments of the present invention relate to a chemical vapor deposition (CVD) system for semiconductor fabrication and in situ dry cleaning methods using the same.
2. Description of the Related Art
A native oxide typically forms when a substrate surface is exposed to oxygen. Oxygen exposure occurs when the substrate is moved between processing chambers at atmospheric conditions, or when a small amount of oxygen remaining in a vacuum chamber contacts the substrate surface. Native oxides may also result if the substrate surface is contaminated during etching. Native oxides typically form an undesirable film on the substrate surface. Native oxide films are usually very thin, such as between 5 and 20 angstroms, but thick enough to cause difficulties in subsequent fabrication processes.
Such difficulties usually affect the electrical properties of semiconductor devices formed on the substrate. For example, a particular problem arises when native silicon oxide films are formed on exposed silicon containing layers, especially during processing of Metal Oxide Silicon Field Effect Transistor (“MOSFET”) structures. Silicon oxide films are electrically insulating and are undesirable at interfaces with contact electrodes or interconnecting electrical pathways because they cause high electrical contact resistance. In MOSFET structures, the electrodes and interconnecting pathways include silicide layers formed by depositing a refractory metal on bare silicon and annealing the layer to produce the metal silicide layer. Native silicon oxide films at the interface between the substrate and the metal reduce the compositional uniformity of the silicide layer by impeding the diffusional chemical reaction that forms the metal silicide. This results in lower substrate yields and increased failure rates due to overheating at the electrical contacts. The native silicon oxide film can also prevent adhesion of other CVD or sputtered layers which are subsequently deposited on the substrate.
Sputter etch processes have been tried to reduce contaminants in large features or in small features having aspect ratios smaller than about 4:1. However, sputter etch processes can damage delicate silicon layers by physical bombardment. In response, wet etch processes using hydrofluoric (HF) acid and deionized water, for example, have also been tried. Wet etch processes such as this, however, are disadvantageous in today's smaller devices where the aspect ratio exceeds 4:1, and especially where the aspect ratio exceeds 10:1. Particularly, the wet solution cannot penetrate into those sizes of vias, contacts, or other features formed within the substrate surface. As a result, the removal of the native oxide film is incomplete. Similarly, a wet etch solution, if successful in penetrating a feature of that size, is even more difficult to remove from the feature once etching is complete.
Another approach for eliminating native oxide films is a dry etch process, such as one utilizing fluorine-containing gases. One disadvantage to using fluorine-containing gases, however, is that fluorine is typically left behind on the substrate surface. Fluorine atoms or fluorine radicals left behind on the substrate surface can be detrimental. For example, the fluorine atoms left behind can continue to etch the substrate causing voids therein.
A more recent approach to remove native oxide films has been to form a fluorine/silicon-containing salt on the substrate surface that is subsequently removed by thermal anneal. In this approach, a thin layer of the salt is formed by reacting a fluorine-containing gas with the silicon oxide surface. The salt is then heated to an elevated temperature sufficient to dissociate the salt into volatile by-products which are then removed from the processing chamber. The formation of a reactive fluorine-containing gas is usually assisted by thermal addition or by plasma energy. The salt is usually formed at a reduced temperature that requires cooling of the substrate surface. This sequence of cooling followed by heating is usually accomplished by transferring the substrate from a cooling chamber where the substrate is cooled to a separate anneal chamber or furnace where the substrate is heated.
For various reasons, this reactive fluorine processing sequence is not desirable. Namely, wafer throughput is greatly diminished because of the time involved to transfer the wafer. Also, the wafer is highly susceptible to further oxidation or other contamination during the transfer. Moreover, the cost of ownership is doubled because two separate chambers are needed to complete the oxide removal process.
There is a need, therefore, for a processing chamber capable of remote plasma generation, heating and cooling, and thereby capable of performing a single dry etch process in a single chamber (i.e. in-situ).